upps, sorry - the 404 is my fault for our experimental/unstable repo - i deleted it and forget to announce it
so, if one remove all lines with
deb $httpfoo/[$bar/]/experimental {unstable|next|experimental} main ...
deb-src $httpfoo/[$bar/]/experimental {unstable|next|experimental} main ...
from one's $sources.list's and run a apt-get update, the most of the 404 errors are gone.
Second: There are some bugs in dpkg|apt|$apt-related - most annoying is the bug with Sources not found - this worked for years without changes, the latest updates seems to expect a not packed Sources - until now we provided Sources packed with gz and bzip2 - changed our main Repos yesterday, but left out the kdenext-Repos for now - will change this later.
For the bad header and other things - this must be fixed upstream, this is not special for siduction.